9 man sng's

Hi, question for 9 man sit n gos. When you have a stack of under 20 bbs should you ever just flat call or should you generally just be shoving or folding? Also, for heads up what hands should you choose to 3 bet light with or is the hand of less importance and it should be more focussed on how often your opponent is opening on the button? Help from coaches and good players would be greatly appreciated. Thanks.

Generally, for 20bb, you shouldn't be flatting. It's should be either jamming or foldind. Of course there are exceptions, for e.x. flatting for 16-17bb against an aggressive player who has a very high stealing %, he's aggro post, but will call a push with a very narrow range. Then we gain more value by just flatting and let him spew postflop. Keep in mind that if that's a regular player, he would most probably learn your tendencies and play super careful if you jam all your weaker re-steal range, but flat monsters only.

As for 3betting light in HU, we care about both.

How often does the SB/BTN steal by raising and how often and with what hands he will call on our shove. Stack sizes also matter a lot!

Generally pick blocker hands, like Ax, low suited kings and queens, especially the rag Aces as they play horribly postflop, but it's very likely you have the best hand pre, also small pairs which are great semi-bluffing hands as usually:
-you have fold equity
-you have decent equity when called
Hands like mid-suited connectors/gappers are also great re-stealing hands as they have decent equity when called. F.e. check the equity T8s has against A2o f.e.
It's a coinflip.